Warning, speedsters: you can't fool quantum radar

IS IT a bird, a plane or a speeding car? Only quantum radar can tell. The ultra-secure technology could counter the speed gun jammers installed in some cars, and even block futuristic military cloaking systems that could disguise a warplane as a bird, say.

Radar and lidar systems bounce radio or light signals off an object and measure how long they take to return. That information can be used to determine the object's position and shape, or to calculate its speed. But military and police systems can be fooled by devices such as speed gun jammers, which generate photons of the same frequency as the outgoing beam.

To reveal when returning photons have been faked, Mehul Malik and colleagues at the University of Rochester, New York, borrowed a trick from quantum cryptography. They polarised each outgoing photon in one of two ways according to a sequence.

Their radar system measures the polarisations of the returning photons, forcing anyone wishing to create a false beam to polarise the photons in the same sequence. But if that person tries to measure the photons arriving from the radar transmitter, quantum mechanics ensures that many of the true polarisations get lost. So a false signal always has more wrongly polarised photons than the true beam.

In a lab test, photons reflected from a model of a stealth bomber had an error rate of less than 1 per cent. When the team tried to spoof the photons as if they were reflected off a bird, over half had the wrong polarisation (Applied Physics Letters, doi.org/jz5). The same principle can also be used to reveal if photons encoding a secret key have been intercepted.

Vadim Makarov of the University of Waterloo, Canada, warns that things will be more complex outside the lab as reflected photons are more likely to change polarisation. "If it works in practice, it would be super-cool," he says.

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